Satellites and spacecraft are used widely and can serve a variety of purposes. A satellite may be used for communication purposes between multiple positions on the earth which would not otherwise have a direct means of communication. A satellite or spacecraft may also be used for scientific purposes. the spacecraft may act as a platform for sensors which are either pointed at the earth or out into space.
In order to keep the sensors on a spacecraft pointed in the right direction or antennas pointed to the right place on the earth, an orientation system must be provided. In most cases, a three axis reference system must be used in order to provide the proper orientation. The spacecraft may then be stabilized through a variety of methods such as magnetic torquers, chemical thrusters, moment or reaction wheels, or any combination of three.
The orientation of the spacecraft may be determined by a variety of systems. One system provides an earth horizon sensor which scans the edges of the earth in the visible light range to provide pitch and roll orientation, while a separate sensor, which locates the sun at particular times of the day, provides yaw information. During the periods when the sun measurement is not available, the yaw is estimated. The horizon sensors may not provide accurate information of the pitch and roll because of atmospheric conditions on the earth.
Gyroscopes may be used in conjunction with the horizon sensors to provide yaw information. However, due to mechanical friction and other inefficiencies the gyroscopes must be periodically updated in order to provide accurate information. This may be done through a variety of star sensors and ephemeris information.
Another type of system uses an infrared sensor to scan the earth and locate the centroid. Around the centroid, the pitch and roll of the spacecraft can be determined. The limb of the earth can be scanned in the infrared range, as it provides a stable reference point for finding the centroid. The drawbacks of the infrared sensor are that its weight limits its applicability in many scenarios, its need for a cooling system, and its inability to provide three axis information. In most cases a gyroscope or an additional star must be used to provide yaw information.
It is an object of this invention to provide a three axis orientation system which is light weight and only requires a single sensor.
It is also an object of the invention to provide a method of providing three axis information for a spacecraft using an single ultraviolet sensor.